support for running CASL from the host prep step

[raffaelespazzoli]

some customers may have the infrastructure pre-provisioned.
I think there is value in running CASL also in this scenario.
Ideally we would start the CASL playbook from the host-prep step.
Everything else should be reusable.
We should provide documentation on how to run the playbooks in this scenario.

A way to implement this maybe is to say that there is a new type of hosting infrastructure:
hosting_infrastructure: preprovisioned

and then add the necessary logic to the end to end playbook.

[oybed]

This is being addressed by #237